Crystal structure of bis[4-(allyloxy)-N′-(but-2-en-1-ylidene)benzohydrazidato]nickel(II)

In the title mononuclear nickel(II) complex, the nickel(II) atom is bischelated by a carbohydrazinate ligand bearing unsaturated alkyl chains.


Chemical context
Hydrazones are a specific class of Schiff-base compounds that are distinguished by the presence of a -CO-NH-N pharmacophore group, and exhibit a wide range of biological activity (Khan et al., 2003;Joshi et al., 2008;Terzioglu & Gü rsoy, 2003). Hydrazone molecules display a number of features, such as their degree of flexibility, a conjugatedsystem and an NH unit that readily participates in hydrogen bonding and may be easily deprotonated. In addition, hydrazone molecules behave as bidentate ligands through their carbonyl oxygen and azomethine nitrogen atoms, and are widely used in coordination chemistry for their ability to form complexes with metal ions in variable oxidation states (Abou-Melha, 2021;Abser et al., 2013;Saygıdeg er Demir et al., 2021;Gond et al., 2022;Velá squez et al., 2020). In this respect, the formation of metal complexes plays an important role in enhancing the biological activity of hydrazones . In addition, providing the molecule with additional donor sites in this type of ligand can modulate the nuclearity of complexes (Vrdoljak et al., 2023). As part of our studies in this area, this paper describes the crystal structure of a bis-[benzohydrazidato]nickel(II) complex.

Structural commentary
The nickel(II) cation of the title complex, [Ni(C 14 H 15 N 2 O 2 ) 2 ], is located on a crystallographic inversion centre and exhibits a square-planar coordination geometry, with a trans configuration of the N,O-chelating ligands, as imposed by the crystal symmetry. An ellipsoid plot of the complex is shown Fig. 1. The structural characterization revealed that the complex is disordered over two orientations ( Fig. 2) with refined occupancies of 0.898 (2) and 0.102 (2). As a result of the low percentage of the second component, the discussion is limited to the species at higher occupancy ( Fig. 1). The Ni-O and Ni-N bond lengths are 1.8432 (16) and 1.8596 (18) Å , respectively, and the O1-Ni-N1 chelating angle is 84.13 (7) . The C2-C3 and C13-C14 bond lengths are 1.319 (4) and 1.258 (5) Å , respectively, which confirm their double bond character (Allen et al., 1987). Intramolecular C4-H4Á Á ÁO1 and C11a-H11aÁ Á ÁO1a interactions (Table 1), where the CÁ Á ÁO distances are 2.975 (3) and 2.801 (3) Å , respectively, reinforce the crystal structure.
The X-ray diffraction analysis revealed that non-hydrogen atoms of the ligand are nearly coplanar; the maximum deviations being 0.308 (3) and 0.313 (5) Å for the allyl carbon atoms C13 and C14, respectively, on either side of the molecular mean plane. The five-and six-membered rings form a dihedral angle of 7.5 (2) . This conformation, which is rather common for this type of molecule (Al-Qadsy et al., 2021;Al Banna et al., 2022;Krishnamoorthy et al., 2012), allows for electron delocalization throughout the molecule.

Supramolecular features
Despite the presence of phenyl rings in the ligands, there is no evidence ofstacking. The crystal packing is, however, supported by unconventional hydrogen bonds of type C-HÁ Á ÁO, e.g. C8-H8Á Á ÁO2(Àx + 1, Ày + 1, Àz + 1) that connect complexes to form ribbons in the [111] direction (Fig. 3, Table 1). In addition, C-HÁ Á Á interactions are realized by centrosymmetrically related complexes (HÁ Á Áphenyl centroid distance = 2.88 Å , Table 1) and give rise to a polymeric chain in the crystallographic [011] direction (Fig. 4). These interactions form a di-periodic architecture, as depicted in Fig An ellipsoid plot (probability at 50%) of the Ni II complex with atom labels for the crystallographically independent part.

Figure 2
The two disordered species in the crystal with occupancies of ca 0.90/0.10. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
To a solution of 4-(allyloxy)benzohydrazide (0.514 g, 2.6 mmol in 20 mL of ethanol), crotonaldehyde (0.187 g, 2.6 mmol) was added and the mixture was refluxed for an hour. Then a solution of nickel(II) acetate tetrahydrate (0.335 g, 1.3 mmol in 10 mL of ethanol) was added and refluxing was continued for an additional two hours. The resulting orange precipitate was filtered off and washed with hot ethanol. The product was recrystallized from a mixture of chloroform and toluene (1:1, v/v), and orange crystals, suitable for X-ray diffraction, were formed. Yield: 0.44 g, 60%, melting point: 511-513 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The structure is disordered, having a second component with a low occupancy of about 10%. The whole component at lower occupancy was refined with DELU and RIGU restraints, with bond lengths restrained to those at higher occupancy by use of the instruction SAME (Sheldrick, 2015b). The hydrogen atoms were included at idealized positions, using a riding model with fixed isotropic displacement parameters [C-H = 0.95-0.99 Å ; U iso (H) = 1.2 or 1.5 U eq (C)].

Figure 4
Detail of the crystal packing showing C-HÁ Á Á interactions, forming a mono-periodic chain in the [011] direction.

Bis[4-(allyloxy)-N′-(but-2-en-1-ylidene)benzohydrazidato]nickel(II)
Crystal data [Ni(C 14  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (